1. Field of the Invention
The present invention relates to a multi-color offset printing method and apparatus.
2. Description of the Related Art
A multi-color offset printing apparatus performs multi-color offset printing by feeding color inks to a plurality of corresponding plates mounted on a plate cylinder, transferring the inks to a blanket cylinder, and thereafter transferring the inks from the blanket cylinder to printing paper in a predetermined order of colors.
Such a multi-color offset printing apparatus has a draw-back that, at a stage immediately following the start of a printing operation, print quality is destabilized by a phenomenon known as xe2x80x9cback trappingxe2x80x9d.
Specifically, in a multi-color offset printing apparatus for printing with four color inks, for example, blankets corresponding to the four color inks successively contact printing paper to produce multi-color prints. After the first color ink is transferred from its blanket to the printing paper, the printing paper successively contacts the other blankets to receive the second, third and fourth color inks therefrom. Then, the first color ink is transferred from the printing paper back to the blankets of the second, third and fourth color inks. Similarly, the second color ink is transferred from the printing paper back to the blankets of the third and fourth color inks as these blankets successively contact the printing paper to transfer the third and fourth color inks. Further, the third color ink is transferred from the printing paper back to the blanket of the fourth color ink as this blanket contacts the printing paper to transfer the fourth color ink.
Consequently, by the time a four color printing operation is completed, the first to third color inks have been applied to the printing paper in smaller quantities than are required to attain a desired printing result.
In printing on a plurality of sheets of printing paper, such a phenomenon continues until the second to fourth blankets become saturated with the inks as a result of the first to third color inks transferred back thereto in certain quantities. Thus, numerous sheets of printing paper are wasted before the blankets are saturated with the inks.
To solve this problem, Applicant has proposed a multi-color offset printing method disclosed in U.S. Pat. No. 4,854,232. According to this method, a state of saturation may be created before starting a printing operation, by transferring beforehand each preceding color ink from its plate to a blanket or blankets of a succeeding color or colors.
However, this multi-color offset printing method is applicable only to a printing apparatus constructed for causing the printing plate of each preceding color to contact the blankets of succeeding colors. It is impossible to apply this method to a printing apparatus having no such construction.
The object of the present invention, therefore, is to provide a multi-color offset printing method and apparatus effective to prevent a wasteful use of printing paper occurring immediately after the start of a printing operation.
The above object is fulfilled, according to the present invention, by a multi-color offset printing method for performing multi-color printing by feeding color inks, respectively, to a plurality of corresponding plates mounted on a plate cylinder, transferring the color inks through a blanket cylinder to printing paper in a predetermined order of colors, the method comprising an ink feeding step for feeding the color inks from ink feeders opposed to the plate cylinder with plates mounted peripherally thereof and defining n (n being an integer 2 or more) image areas, the color inks fed corresponding in color to the image areas, respectively; an ink transferring step for transferring the color inks from the plates to the blanket cylinder equal in diameter to the plate cylinder and rotating in contact with the plate cylinder; an ink adjusting step for rotating an impression cylinder at least 2 n times, the impression cylinder having a sheet mounted peripherally thereof and rotating in contact with the blanket cylinder; and a printing step for feeding printing paper to an outer periphery of the impression cylinder rotating in contact with the blanket cylinder, and discharging the printing paper from the outer periphery of the impression cylinder after rotating the impression cylinder n times.
With this multi-color offset printing method, a state of ink saturation is created on the blanket cylinder prior to a printing operation. This feature is effective to prevent a wasteful use of printing paper occurring immediately after the start of a printing operation.
In a preferred embodiment of the invention, the ink transferring step is executed for causing only one of the image areas corresponding to a succeeding one of the color inks in the order of colors to contact the blanket cylinder.
In another aspect of the invention, a multi-color offset printing method is provided for performing multi-color printing by feeding color inks, respectively, to a plurality of corresponding plates mounted on plate cylinders, transferring the inks through blanket cylinders to printing paper in a predetermined order of colors, the method comprising a first ink feeding step for feeding the color inks from ink feeders opposed to a first plate cylinder with a plate mounted peripherally thereof and defining m (m being an integer 1 or more) image areas, the color inks fed corresponding in color to the image areas, respectively; a second ink feeding step for feeding the color inks from ink feeders opposed to a second plate cylinder with a plate mounted peripherally thereof and defining m image areas, the color inks fed corresponding in color to the image areas, respectively; a first ink transferring step for transferring the color inks from the plate mounted peripherally of the first plate cylinder to a first blanket cylinder equal in diameter to the first plate cylinder and rotating in contact with the first plate cylinder; a second ink transferring step for transferring the color inks from the plate mounted peripherally of the second plate cylinder to a second blanket cylinder equal in diameter to the second plate cylinder and rotating in contact with the second plate cylinder; an ink adjusting step for rotating an impression cylinder at least 2 m times, the impression cylinder having a sheet mounted peripherally thereof and rotating in contact with the first blanket cylinder and the second blanket cylinder; and a printing step for feeding printing paper to an outer periphery of the impression cylinder rotating in contact with the first blanket cylinder and the second blanket cylinder, and discharging the printing paper from the outer periphery of the impression cylinder after rotating the impression cylinder m times.
In a further aspect of the invention, a multi-color offset printing apparatus is provided for performing multi-color printing by feeding color inks, respectively, to a plurality of corresponding plates mounted on a plate cylinder, transferring the color inks through a blanket cylinder to printing paper in a predetermined order of colors, the apparatus comprising a plate cylinder with plates mounted peripherally thereof and defining n (n being an integer 2 or more) image areas; a blanket cylinder equal in diameter to the plate cylinder and rotatable in contact with the plate cylinder; an impression cylinder having a gripper disposed peripherally thereof for holding a forward end of printing paper, the impression cylinder being rotatable in contact with the blanket cylinder; and a controller for rotating the impression cylinder at least 2 n times, with the gripper holding a sheet.
Other features and advantages of the present invention will be apparent from the following detailed description of the embodiments of the invention.